Added infrastructure for field cvs. In field cv formalism the bias

is an overlap integral between the current field and some reference
field ( this can be a sum of fields visited at previous times ala metadynamics ).
To test the derivatives of the field I changed calculateNumericalDerivatives
this routine can now be passed an ActionWithValue so that you can differentiate
objects where something other than a value is passed between Actions.  Currently
only histogram style field cvs are implemented.
Wrote documentation for FIELD keyword and for MOMENT keyword.  Also
rethought numbered keywords and so changed the keywords class a little bit
Changed number of decimal places in tests so that they worked on other
computers.  Also made test 23 much less expensive
Fixed a very persisent bug in field cvs - horay!!!